| Abstract
| - The hygroscopic nature of atmospheric aerosol hasgenerally been associated with its inorganic fraction. Inthis study, a group contribution method is used to predictthe water absorption of secondary organic aerosol(SOA). Compared against growth measurements of mixedinorganic−organic particles, this method appears toprovide a first-order approximation in predicting SOA waterabsorption. The growth of common SOA species ispredicted to be significantly less than common atmosphericinorganic salts such as (NH4)2SO4 and NaCl. Using thisgroup contribution method as a tool in predicting SOA waterabsorption, an integrated modeling approach is developedcombining available SOA and inorganic aerosol modelsto predict overall aerosol behavior. The effect of SOA onwater absorption and nitrate partitioning between the gasand aerosol phases is determined. On average, it appearsthat SOA accounts for approximately 7% of total aerosolwater and increases aerosol nitrate concentrations byapproximately 10%. At high relative humidity (≥85%) andlow SOA mass fractions (<20% of total PM2.5), the role ofSOA in nitrate partitioning and its contribution to totalaerosol water is negligible. However, the water absorptionof SOA appears to be less sensitive to changes inrelative humidity than that of inorganic species, and thusat low relative humidity (∼50%) and high SOA mass fractionconcentrations (∼30% of total PM2.5), SOA is predictedto account for approximately 20% of total aerosol water anda 50% increase in aerosol nitrate concentrations. Thesefindings could improve the results of modeling studies whereaerosol nitrate has often been underpredicted.
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